In recent years, according to directives for environmental protection, such as RoHS (Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment) directive and ELV (End of Life Vehicles) directive, it has been required to regulate the use of harmful substances (such as lead, mercury, cadmium, and hexavalent chromium).
To this end, chromate coatings, which are effective as chemical conversion coatings for anticorrosion of members having metal surfaces, such as zinc-plated members, have been formed through the use of a chemical conversion treatment liquid that contains trivalent chromium rather than the use of a composition for chemical conversion treatment using a chromate salt that contains hexavalent chromium (a composition for chemical conversion treatment is also referred to as “chemical conversion treatment liquid” here and hereinafter). The chemical conversion coating obtained using a conventional hexavalent chromium-containing chemical conversion treatment liquid contains soluble hexavalent chromium in the coating. Therefore, such a coating is a target of regulations according to the above directives.
Chromate using a trivalent chromium-containing chemical conversion treatment liquid has become general in such a way, but the trivalent chromium-containing chemical conversion coating obtained through the use of the trivalent chromium-containing chemical conversion treatment liquid is to be formed substantially on a zinc-based metal surface, and cannot used for an iron-based member (a member having an iron-based metal surface). That is, if the trivalent chromium-containing chemical conversion treatment liquid is caused to contact with an iron-based member, then the iron-based material at the surface of the member is rapidly corroded to generate red rust on the iron-based member.
This problem is particularly serious if the member having a zinc-based metal surface is configured such that the base material is provided as an iron-based member having a complex shape (typical examples thereof include secondary products, such as brake caliper and screw.) and plating of a zinc-based metal material (referred also to as “zinc-based plating”) is performed on the surface thereof. The thickness of the zinc-based plating formed on the base material may considerably vary due to complexity of the shape. If, in this case, the trivalent chromium-containing chemical conversion treatment liquid is caused to contact with that member, then a part having a particularly thin thickness of the zinc-based plating will be subjected to an insufficient reaction that the chemical conversion treatment liquid less dissolves the zinc-based material at the surface for dissolution of zinc. This may lead to insufficient deposition, which is considered as a counterpart reaction of the dissolution of zinc, of a substance (such as oxide of chromium) that forms the chemical conversion coating, and may consequently progress a reaction that the iron-based metal surface of the base material is eaten by the chemical conversion treatment liquid. In such a case, the chemical conversion coating may be formed on a part of the member, but the part having a thin thickness of the zinc-based plating will be suffered from the occurrence of red rust. Such a member will be a defective product, which may deteriorate the productivity of products.
Patent Literature 1 discloses, as a technique of forming a chemical conversion coating on an iron-based member, a process for preparing zirconium-chromium chemical conversion coatings on iron and iron alloys to improve the corrosion resistance and adhesive bonding strength, the process comprising treating iron and iron alloys with an acidic aqueous solution having a pH ranging from about 2.5 to 5.5, the acidic aqueous solution comprising, per liter of solution, from about 0.01 to 22 grams of a trivalent chromium compound, about 0.01 to 12 grams of a hexafluorozirconate, about 0.0 to 12 grams of at least one fluorocompound selected from the group consisting of tetrafluoroborate, hexafluorosilicate and mixtures thereof, from about 0.0 to 10 grams of at least one divalent zinc compound, from 0.0 to about 10 grams of at least one water-soluble thickener and from 0.0 to about 10 grams of at least one water-soluble surfactant.
Patent Literature 2 discloses a trivalent chromium chemical conversion coating treatment agent which is aqueous solution for forming a trivalent chromium chemical conversion coating on a metal surface and which contains: 1) a trivalent chromium compound and a cobalt compound; 2) at least one ion supply source selected from chlorine ion, nitrate ion and sulphate ion; and 3) organic acid or organic acid salt, and further contains: 4) at least one selected from allylamine, polyallylamine, aromatic sulfonic acid, aromatic sulfonic acid-formaldehyde condensate, and a derivative thereof. According to the literature, it is said that containing components such as allylamine allows a trivalent chrome chemical conversion coating to be formed even in a case where the pH is 2.5 or more and the etching to the metal base material surface is very weak.